Six million Jews – God’s chosen people – have been shot or gassed. It was a mass murder of truly colossal proportions that could have been easily prevented. But God did not save His chosen people. Why?
For the very same reason – God respects the human Freedom of Will unless the very existence of the Christian civilization is at stake. True, Holocaust was a horrible crime against humanity, but it did not constitute an existential threat to the Church and the Christian civilization.
But the Bolshevist Soviet Union was. It was a very much existential threat to God’s Church (Christian Church) and the whole Christian Civilization. In fact, to the whole human civilization which the Bolsheviks (unconsciously, of course, but it made no difference whatsoever) would have transformed into a very literal Hell on Earth.
And the only power in the world capable of stopping the Bolsheviks in their tracks was the German Wehrmacht. Wehrmacht + Waffen-SS, to be more precise. The Armed Forces of Nazi Germany. Of NAZI Germany.
Hence, God only wanted the Nazis to (1) prevent the Bolshevist Soviet Union from destroying His Church and His civilization; and (2) not do anything that might constitute the existential threat to “both of the above”. As long as they did what He wanted them to do and did not do what He did not want them to do, he did not care what else they did.
So He did not intervene and allowed the Nazis to exterminate many millions of Russians, Poles, etc. In other words, to commit colossal blunders. Because, again, although these mass murders were horrible war crimes, they did not constitute an existential threat to the Church and the Christian civilization. And thus did not bother God in the least.
Horrible? Yes. Inhuman? Absolutely. However, God has His own logic and His own system of priorities which more often than not are very different from ours (i.e. human).
Obviously, we do not like it – from our human perspective our God is far from perfect (to put it mildly). But He is the only God we have so there is nothing we can do about it. Nothing whatsoever.